Medical applications of biopolymeric membranes are manifold. See, e.g., Shu-Tung Li, Biologic Biomaterials: Tissue-Derived Biomaterials (Collagen). In: Biomedical Engineering Handbook, Ed. J. D. Bronzino, 627-647, CRC Press, Inc. Boca Raton, Fla., 1995.
Biopolymeric membranes, such as collagen membranes, can be made by air-drying a biopolymeric fibers-containing solution, or applying an acid or a base solution of dispersed biopolymeric fibers on a flat surface. Li disclosed in U.S. Pat. No. 5,206,028 a method of preparing a collagen membrane by first freeze-drying a collagen dispersion to form a sponge, which is then humidified, compressed, and subjected to chemical crosslinking. Chu et al., on the other hand, disclosed in U.S. Pat. No. 4,725,671 a method of preparing a gel from an atelocollagen solution and then compressing and air-drying the gel to form a collagen membrane.
The biopolymeric fibers in sheet membranes prepared by the prior art methods are randomly oriented. Such membranes generally have low mechanical strength and are only useful in applications where mechanical strength of the device is not a critical factor for function. They are not suturable and tend to tear with a slight suture tug. As most soft tissue enforcement materials require extensive mechanical strength so that they can be easily secured in place either by using sutures, staples, tags, or screws, mechanical strength becomes a critical factor in designing biopolymeric fiber-based membranes for applications in soft tissue repair.
An aspect of this invention relates to a sheet membrane containing at least one layer of oriented biopolymeric fibers, such as collagen fibers. What is meant by xe2x80x9corientedxe2x80x9d is that at least half of the biopolymeric fibers are in one general direction (i.e., xe2x80x9cfiber orientationxe2x80x9d) as determined by the method described below or by an analogous method. The sheet membrane is generally flat but, if desired, can be somewhat curved. It has a thickness of 0.1 mm to 3.0 mm (preferably, 0.2 mm to 1.0 mm), a density of 0.1 g/cm2 to 1.2 g/cm3 (preferably, 0.2 g/cm3 to 0.8 g/cm3), a hydrothermal shrinkage temperature of 50xc2x0 C. to 85xc2x0 C.(preferably, 55xc2x0 C. to 70xc2x0 C.), a suture pullout strength (both perpendicular and parallel to the fiber orientation) of 0.1 kg to 5 kg (preferably, 0.3 kg to 3 kg), and a tensile strength of 10 kg/cm2 to 150 kg/cm2 (preferably, 30 kg/cm2 to 80 kg/cm2), and is permeable to molecules having molecular weights of 200 to 300,000 daltons (preferably, 1,000 to 50,000 daltons). The above recited parameters can be readily measured by methods known to a person of ordinary skill in the art, some of which are described in detail below.
When a sheet membrane is made of two or more layers of oriented biopolymeric fibers, the layers are secured to each other by fibrin glue, collagen glue (gel or moist collagen sponge), suture (resorbable or nonresorbable), crosslinking formation, or the like. Preferably, the biopolymeric fibers in different layers are respectively oriented in different directions.
Another aspect of this invention relates to a method of making a single-layer sheet membrane of oriented biopolymeric fibers. The method includes: (1) reconstituting biopolymeric fibers, e.g., collagen fibers, dispersed in a solution; (2) placing the reconstituted biopolymeric fibers around a mandrel; (3) rotating the mandrel to convert the reconstituted biopolymeric fibers on the mandrel into a tubular membrane of oriented biopolymeric fibers; (4) cutting the tubular membrane longitudinally after it has been dried on the mandrel; (5) rolling the cut membrane into a tubular form that is an inversion of the tubular membrane; (6) inserting the rolled membrane into a tubular mesh; and (7) crosslinking the biopolymeric fibers to form a sheet membrane of oriented biopolymeric fibers.
Various medical uses of the sheet membranes of this invention are described below. Other features or advantages of the present invention will be apparent from the following drawing and detailed description of the invention, as well as from the appending claims.